<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
   <div id="app">
       <div id="inner"></div>
   </div>
</body>
<script>
    window.onload=function () {
        var app = document.getElementById("app");
        console.log(app)

        var inner = app.getElementById("inner");
        console.log(inner)
    }
</script>
</html>